Avian malaria is most notably caused by Plasmodium relictum, a protist that infects birds in tropical regions. There are several other species of Plasmodium that infect birds, such as Plasmodium anasum and Plasmodium gallinaceum, but these are of less importance except, in occasional cases, for the poultry industry.
Its vector is the mosquito Culex quinquefasciatus, which was introduced to the Hawaiian islands. Since arriving, this disease has devastated the native bird population. Virtually every individual of endemic species below 4000 feet in elevation has been eliminated by the disease, as that matches the range of the mosquito. These mosquitoes are limited to lower elevations by their requirement for warmth. However, they appear to be slowly gaining a foothold at higher elevations and their range may be expanding upwards. If so, remaining endemic bird populations may adversely affected.
Most of the Hawaii islands have a maximum elevation of around 5,000 ft, so with the exception of the Big Island and East Maui, native birds may become extinct on every other island if the mosquito is able to occupy higher elevations.
Disease process & epidemiology
Plasmodium relictum reproduces in red blood cells. If the parasite load is sufficiently high, the bird begins losing red blood cells, causing anemia (USDI and USGS 2005). Because red blood cells are critical for moving oxygen about the body, loss of these cells can lead to progressive weakness and, eventually, death (USDI and USGS 2005). Malaria mainly affects birds in the order Passeriformes (perching birds). In Hawaiʻi, this includes most of the native honeycreepers and the Hawaiian crow. Susceptibility to the disease varies between species, for example, the ʻiʻiwi is very susceptible to malaria while the ʻapapane less so (USDI and USGS 2005). Native Hawaiian birds are more susceptible than introduced birds to the disease and exhibit a higher mortality rate (Van Riper et al. 1982; Atkinson et al. 1995). This has serious implications for native bird faunas (SPREP) with P. relictum being blamed for the range restriction and extinctions of a number of bird species in Hawaii, primarily forest birds of low-land forests habitats where the mosquito vector is most common (Warner 1968; Van Riper 1991; USDI and USGS 2005).
The only way to control the effects of avian malaria is to control mosquito populations. This is difficult particularly in remote areas in the wet forests of Hawaii where wallows from feral pigs and hollowed out logs of the native apuu ferns provide ample areas of standing water where the mosquito breeds (USDI and USGS 2005). One effective procedure is to reduce the number of potential water catchment containers in order to reduce the mosquito breeding sites available (SPREP Undated). However, in Hawaii attempts to control the mosquitoes by larval habitat reduction and larvicide use have been largely unsuccessful.